Refractories are materials having properties that make them suitable for use as heat-resistant barriers in high temperature applications. Unshaped refractory materials have the ability to form joint-less linings and are often referred to as monolithics. These materials are useful for example as linings for cupola hearths and siphons, blast furnaces, main, secondary and tilting runners, and more generally vessels or vessel spouts, ladles, tundishes, reaction chambers and troughs that contain, direct the flow of, or are suitable for facilitating industrial treatment of liquid metals and slags, or of any other high temperature liquids, solids or gases.
Dry vibratable mixes (DVM) generally consist of a dry mix of aggregated particles (aggregate size up to 15 mm) and powdered particles (particle size below 100 μm). Spinel forming DVMs consist of alumina rich aggregates (such as white fused alumina; WFA) and fine MgO-powder. DVMs are installed in their final position of use and compacted manually, mechanically, or most commonly by vibration. Refractory linings resulting from such compaction of a DVM are then heated to their final service temperature, which results in a ceramic transformation (sintering, creation of new phases) within the composition. In the case of refractory linings, a gradient of physical properties exists within the linings themselves, caused by a temperature gradient during the firing step, as the lining is typically fired by introduction of molten metal into a recipient covered by the lining, and the lining is therefore only heated by thermal conduction from one (internal) side.
For DVMs, this often leads to a structure of a refractory lining wherein the internal (more strongly heated) portion of the lining is strongly sintered, wherein the more externally located portions are less strongly sintered. Such a gradient of mechanical properties for example prevents cracks, often caused by mechanical or thermal shock on the internal surface of a lining, from propagating through the entire width of the lining. Liquid metal or slag could migrate through the lining by following the volume opened by a crack. Providing a refractory lining which has reduced risk of cracking presents an considerable safety improvement for the hardware lined with dry vibratable mixes.
Spinel forming DVMs, such as alumina-magnesia spinel forming DVMs, are commonly used for forming monolithic refractory linings, since they offer additional advantages in terms of lining performance and service duration. Additional improvement results from the ability of alumina-magnesia spinels to trap iron oxides present in slag within their crystal lattice, thus efficiently preventing slag penetration within the lining and reducing wear by chemical reaction between the refractory and the slag. An additional advantage of the spinel forming DVM is that a volume increase occurs during spinel formation, since the density of spinel is lower than that of powdered alumina and magnesia. This volume increase helps compensate for sintering shrinkage and, depending on quantity of spinel formed, results in further densification, i.e. reduction of porosity in the refractory lining, since volume expansion must be accommodated within the refractory microstructure as the lining operates in a volume-restricted environment.
Therefore, it is commonly accepted in the art that overall performance of a refractory lining is directly linked to the quantity of spinel formed during heating up and operation of the refractory DVM spinal.
Spinel forming DVMs according to the state of the art comprise fine alumina powder (particle diameter <0.1 mm), fine MgO powder (particle diameter <0.1 mm) and coarse alumina grains and aggregates (0.1 mm<particle diameter <15 mm). A particular ratio between the powder and aggregate fractions must exist, so that the product can be installed and compacted in the intended space. If the fraction of fine powders becomes too high, a DVM cannot be installed efficiently. Spinel forming DVMs according to the state of the art therefore have an MgO content no higher than 15 wt.-%. However, the stoechiometric ratio for forming alumina-magnesia spinel is 71.8 wt.-% Al2O3 to 28.2 wt.-% MgO.